There was a bonus for Birmingham City, drawn last night to be Portsmouth's FA Cup quarter-final opponents, in that they may yet receive a bye to the last four. Despite the protestations of the manager Avram Grant and David James, financial come-uppance could still be at hand for the profligacy involved in signing players like the England goalkeeper to help win the Cup two years ago.

The club must produce evidence by Wednesday that they are not trading insolvently, and then have to return to court a fortnight today, five days before the scheduled sixth-round tie. For now they are not so much drinking in the last-chance saloon as staging a lock-in there. Publicans in Portsmouth must have been rubbing their hands awaiting the return of the 4,000 loyalists who had travelled along the M27 to St Mary's. Southampton hostelries, meanwhile, were advised to close for two hours after the game, which was bad for business since there were sorrows to be drowned by more than 27,000 home supporters.

The taunts must have been hard to bear for them, not least because the scoreline became a grossly flattering one as their team, mid-table in League One after a 10-point deduction, were repeatedly caught on the break in conceding four goals in the last 20 minutes of what eventually became a rousing tie.

A year ago, it might have been labelled the debtors' derby. By Saturday, bankrolled by the ambitious Swiss supporter Markus Liebherr, Southampton were even able to parade a gloating fake sheikh in the main stand, waving "loadsamoney" at the impoverished visitors. The stand was rocking at that stage, Rickie Lambert, a lively striker signed from Bristol Rovers with Liebherr's backing, having scored his 24th goal of the season to equalise Quincy Owusu-Abeyie's opener.

The latter's pace down the left, however, was to prove a decisive factor after Grant brought him on for Angelos Basinas, a liability as a holding midfield man, and changed Portsmouth's shape. Fortunes changed too as two perfect counter-attacks and a corner brought goals for Aruna Dindane, Nadir Belhadj and Jamie O'Hara, a player, like Quincy, that the club have been allowed to bring in on loan despite their transfer embargo.

So a rare away win resulted in a derby that had previously produced seven straight home victories and no Portsmouth success on enemy territory since 1988. Owusu-Abeyie and O'Hara both had important parts to play in it and the one other critical stroke of fortune Grant has enjoyed since succeeding Paul Hart was that David James's proposed move to Stoke City last month broke down. "I said again and again that I wanted David James to stay in the team and now you can see why," Grant said. "In the last game [against Sunderland] we played brilliantly, we created a lot of chances, but at 1-0 he saved a 100 percent goal and then we equalised. Today when it was 0-0, great saves – that is Jamo. He played three times in a week, and he trains as he needs to train. He's a top professional."

James, who has forfeited a lucrative new contract that he was entitled to, said after the game that he did not believe a Premier League club would be allowed to go to the wall as they are "too important to too many people". Grant suggested: "I think football is not just about money and contracts. I think if players and coaches and owners think only about this they will not succeed. I don't believe that you should see football only as a business." Many will agree, but Portsmouth desperately need one High Court judge to do so.

Given the hyper-tense nature of the clubs' first meeting for five years – the last one also ended 4-1 to Portsmouth – the players conducted themselves well, with only three bookings and one brief altercation. Outside, however, there were reports of bricks, chairs and bottles being thrown and police said there may be further arrests to add to Saturday's 11.